1. Field of the Invention
This invention relates to the field of electrostatography and, more particularly, to improvements in apparatus for controlling toner replenishment.
2. Description of Prior Art
In electrostatography, electrostatic images formed on a dielectric recording element are rendered visible via the application of pigmented, thermoplastic particles known as toner. Typically, such toner forms part of a two-component developer mix consisting of the toner particles and magnetically-attractible carrier particles to which the toner particles adhere via triboelectric forces. During the development process, the electrostatic forces associated with the latent image act to strip the toner particles from their associated carrier particles, and the partially denuded carrier particles are returned to a reservoir.
It is well known in the art to continuously monitor the toner concentration in an electrostatographic developer mix and to replenish the mixture with toner when the concentration thereof falls below a predetermined level. Such a toner concentration monitor can be easily calibrated to compensate for toner depletion from the development system regardless of cause. Its significant drawback is that it is relatively slow to respond to abrupt changes in toner depletion rate, such as occasioned by a change in the image content of the documents being printed from ones having little image information thereon, to ones having large solid or continuous tone image areas. Typically, several minutes will elapse before the toner concentration is restored to a level at which copies of a desired image density can be obtained. According to U.S. Pat. No. 4,468,112, such methods are unable to maintain the toner concentration at an appropriate level over a prolonged period, as they are apt to be influenced by the spent toner particles which are present in the developer but do not contribute to the developing process. Also such methods, not based on the direct measurement of the developed image density, are unable to compensate for changes in the characteristics of the recording element due to atmospheric conditions and prolonged use.
It is also known in the art to continuously monitor toner depletion from an electrostatographic development station by monitoring the amount of toner applied to the recording member during development. For example, in the commonly assigned U.S. Pat. No. 3,674,353 issued to Trachtenberg, a pair of induction plates, positioned adjacent the recording member on the upstream and downstream sides of the development station, function to sense the overall charge on the recording member before and after development. The difference in charge induced on the plates by the passage of the undeveloped and developed charge patterns has been found to be an accurate measure of the quantity of toner depleted from the development station. A toner depletion signal, proportional to the difference in charge induced on the induction plates, is used to control toner replenishment.
Another method for continuously monitoring toner depletion from a development station is useful in electronic printers. The replenishing rate is adjusted in response to the number of character print signals applied to the print head. The print signals may be in character code and a statistical average take-out rate used to estimate toner depletion, or the signals may be picture elements (pixel) signals. See for example U.S. Pat. Nos. 3,529,546 and No. 4,413,264.
While such toner depletion monitors are quicker to respond than are toner concentration monitors, their use for controlling toner replenishment has certain disadvantages. Any toner depletion aside from that caused by image development, such as for example toner depletion caused by dusting and other losses, is not sensed by a depletion monitor, and hence will not be replenished. Nor can such a monitor detect and cure inaccuracies or defects in the toner replenishment process itself. In short, toner depletion monitors are difficult, at best, to calibrate for precise control of toner replenishment.
Co-pending, commonly assigned U.S. patent application Ser. No. 052,632 filed in the name of William A. Resch, III on May 21, 1987, discloses a toner replenishment control apparatus which overcomes the aforementioned disadvantages of prior art systems. A toner depletion signal is produced having a value indicative of the rate of toner usage. A replenishment controller actuates toner replenishment proportionally in accordance with the value of the depletion signal. A second signal is produced having a value proportional to toning contrast; and the constant of proportionallity between the toner depletion signal and the replenishment is adjusted according to the second signal.
While the control apparatus disclosed in said Application Ser. No. 052,632 is excellent for electrostatographic machines having traditional developer materials which exhibit toning contrast characteristics which vary predictably with the concentration of toner particles in the developer mix, new developer materials are becoming commercially available which do not exhibit predictable changes in toning contrast as the toner concentration changes.
Fortunately, the toning contrast characteristics of these new developer materials do not exhibit large changes with changes in environmental conditions, and it is therefore not necessary to adjust the replenishment rate according to the toning contrast characteristics for these materials. However, it is likely that both traditional developer materials and the new type of developer materials may be used in the same machine; even for the development of a single reproduction. For example, a color reproduction is formed by developing several color-separation latent images and transferring the developed images in succession to a single receiver sheet. Black portions of the image may be produced by adding red, blue, and green toners in equal amount (called process black) but is more commonly produced by using a fourth color station with black toner (called toner black). If the red, blue, and green toner was of the traditional type and the black toner was of the new type, the replenishment scheme for the different material types must be different for optimum results.